Officials say they had no intelligence beforehand that a massive terrorist plot was under way.

There have been a number of denials of responsibility by Palestinian groups and by the al Qaeda group headed by fugitive Saudi accused terrorist Osama bin Laden. U.S. officials say they have no credible claim of responsibility.

A spokesman for the Central Intelligence Agency said the CIA operations center has been moved to an undisclosed location. The headquarters building in Langley, Virginia, has been evacuated of all but essential personnel.

A spokesman at the National Security Agency in Fort Meade, Maryland, said the NSA has a "heightened security posture" and that all non-essential personnel have been sent home.

U.S. intelligence officials say they are contacting all their sources and going through intelligence intercepts searching for evidence as to who may have organized the attacks.

"We are looking for any shred of information that could help," an official said.